This invention relates to inductive low pressure radio frequency plasma reactions, and in particular to a plasma process for the deposition of glassy material onto a solid surface.
The inductive radio frequency plasma has been known for many years. The essential feature of the inductive discharge is that the power is introduced into the gas phase by inductive coupling and hence the conductor paths in the gas form closed paths within the container. This provides a hot intense plasma and has the advantage that no internal electrodes are required nor are there the problems with large potential drops, as can occur with capacitive coupling, at the walls of the containing vessel.
The term `radio frequency` as used herein is understood to include microwave frequencies.
An inductive discharge, or H-discharge, is produced by the magnetic field (H) of the exciting coil, unlike a capacitive discharge or E-discharge which is relatively diffused and is produced by electrostatic fields. It has been found that the H & E discharges become indistinguishable as the wavelength of the exciting radiation becomes comparable with the dimension of the discharge.
At low pressures the discharge tends to be most intense at the walls of a containing tube. At higher pressure 500 Torr the discharge becomes more restricted to the center of the tube.
The use of H-discharges for chemical processing has been limited previously to the atmospheric plasma torch. This device is essentially a high power H-discharge which is generally operated in argon to ease power requirements and at 3-10 MHz. Such a plasma torch has been used in the past to produce ultra-pure silica. The reactants were introduced into the tail flame of the plasma, as oxygen and silicon tetrachloride in high concentration tend to extinguish the plasma. Silica produced by such a torch was in the form of sub-micron spheres which had to be collected and sintered to form clear glass. While such an arrangement has proved effective for performing many chemical reactions it does not lend itself readily to the production e.g. of optical fiber preforms, in which it is preferred to deposit material directly as a glassy layer so as to avoid an intermediate sintering process.